This invention relates in general to datuming devices using a displacement probe and, more particularly, to datuming devices which are capable of evaluating displacements and alignments that are not parallel to the probe's axis direction of travel.
At it's present stage of development, the precision machining field continues to develop at a rapid pace toward more complete automation. Systems that are totally dependent on manual operations have largely given way to techniques whereby manufactured parts are made with general purpose, numerically controlled (NC) machining systems. Although cutting or other removal of material occurs automatically in such systems, numerous manual operations are unfortunately still required, primarily for measuring the machined dimensions, and for making cutter adjustments using an ordinary numerical control cutter offset. These manual measurements and adjustments of the cutting tool are necessary to take into account a large number of variables, such as wear of the cutting tool, repositioning and/or replacement of the cutting tool, as well as dimensional changes of the cutting tool, the work piece and the machining apparatus itself due to such factors as heating, deflection under load and the like.
By way of example, in a typical operation carried out with a numerically controlled machine tool such as a lathe, certain adjustments, i.e., tool offsets, must be manually implemented by the operator after the machine is set up for the manufacture of a particular work piece or part. Prior to the start of machining, the operator must advance the cutting tool to a tool setting surface and determine the tool position by manually measuring the space between the tool and the reference surface. This is normally done with a piece of shim material or the like and such measurements then form the basis for manually making the tool offsets. Where the lathe includes tool holding means such as a multiple tool turret, this operation must be carried out separately for each tool, as well as for each of the axes (of motion) of the machine. Prior to making the final or finishing cut for a particular work piece surface, the various dimensions of the semi-finished work piece surface are measured by using a hand-held gauge in order to measure the conformance of the actual dimensions of the finished surface to the desired dimensions.
The manual operations described above are individually time-consuming and take up a significant amount of the total time required to machine a particular work piece to the desired dimensions. This serves to limit the manufacturing capacity of the machine tool. Considering present day costs of a lathe or a milling machine (machining center), any reduction of the capacity of the machine tool becomes a matter of economic significance. Further, all such manual operations are prone to introducing errors into the manufacturing process.
As is generally recognized, the solution to the foregoing problems is to automate manual measurements and the manual adjustments of the cutting tool by the use of a computer-operated numerical control (CNC) system. In such a system, the computer may either be positioned remote from the numerical control unit, or it may be incorporated in the latter in the form of a microcomputer, for example. Instead of down-loading successive blocks of data stored on tape or the like as is the case in an ordinary NC system, a computer numerical control (CNC) system is capable of storing entire programs and calling them up in a desired sequence, editing the programs by addition or deletion of blocks, and carrying out the computations of offsets and the like.
For the CNC system to control a cutting tool or other implement as accurately as possible, it is necessary for the system to know the location of the cutting tool with great precision. To achieve this end, it is known to mount a contact probe or tool sensor on the bed of the machining apparatus, or on a pivotal arm that can be swung out of the way when desired. The position of the cutting tool can be calibrated against such a probe by noting the probing axis scale position when contact with the probe occurs. From the observed deviations between the programmed and actual positions, a compensating offset may be determined and stored in the memory associated with the computer numerical control system. The offset compensates for the difference between the programmed contact position of the cutting tool or other implement and the actual contact position. It is also known to mount a contact probe or part sensor in a tool holding structure; to calibrate such a probe against a reference surface on the machine; to probe the machined surface of the work piece and to derive from such probing the information for determining the final offset required for the finishing cut; and to probe the finished surface for conformance with the desired dimensions.
While displacement probes have been able to accurately measure displacement in the machine axis direction of the probes movement for some time now, apparatus and techniques for measuring displacement in a direction not parallel to the probe's axis motion have heretofore been generally unavailable, difficult to implement or expensive to fabricate. For example, a laser interferometer can be used to measure cross-directional travel but requires special equipment and excessive set-up time.
In CNC machines it is often desirable to measure displacement in directions not parallel to the displacement probe's movement. For example, in a vertical turning lathe, the rotatable bed on which the work piece is situated may exhibit backlash when it is turned clockwise to a predetermined rotational position and then turned counter-clockwise back to that same position. To measure such backlash and compensate for the same, it is necessary to measure displacement in a direction not parallel with probe motion, namely, an angular displacement with respect to the rotatable bed.
Important machine tool alignments and displacement measurements exist which are critical to the machining processing and which have not been automated due to the fact that such alignments and displacements are not parallel to the probe's line of travel or axial feed direction. For example, horizontal lathes and vertical turning lathes do not have "Y" axis or "Y" direction of travel. However, there are important displacements on these machines in the "Y" direction. If the cutting tool point for such a machine is misaligned in the "Y" direction, this can cause undesired chatter, tool rubbing and other erratic cutting conditions. Moreover, if diameters are probed on such machines when the probe is misaligned in the "Y" direction, then improper size measurements will result since chords will be measured instead of diameters.